Tsunami warning Scientists now understand why the 2011 Japanese tsunami was bigger than expected and say we need to prepare for larger tsunamis in the future.

The warning comes following an analysis of the quake which caused the tsunami, reported today in the journal Science.

"Japan did not expect a tsunami that size. The country had a tsunami defence system in place but because it didn't expect such large waves, the system was inadequate," says Dr Virginia Toy, a geologist at the University of Otago in Dunedin, New Zealand, who was involved in the study.

"We now realise we've got to build systems that have greater capacity."

The Tohoku-Oki earthquake off Japan resulted from a collision of the Pacific plate and the plate that Japan sits on.

As continental plates collide, in what's called a subduction zone, stresses build up in the Earth's crust and when these are released, an earthquake occurs.

In the Japan quake this relief of stress caused a 50-metre movement of the Earth's crust leading to a devastating tsunami.

Stress relief

Toy says earthquakes usually only release about one-tenth of the built-up stress.

"One of the reasons that most normal earthquakes only relieve a bit of the stress is because there is some friction on the fault, resisting the earthquake from happening," she says.

In 2012, Toy was part of an expedition that set out to investigate how much heat from friction had built up at the subduction zone.

But while drilling into the crust to install temperature sensors, the researchers made another observation.

Toy says the orientation of fractures radiating from the drill holes suggested that the amount of stress released at the time of the quake was much more than is usually released during earthquakes.

"What's super surprising about this study is that it appears we had almost total stress release," she says. "It is unusual."

Toy says the fact all of the stress was relieved suggests very low friction at the fault. She expects the temperature measurements of the rocks at the subduction zone to reflect this too, once the sensors are read.

"It's very slippery," she says. "And that in part explains why we had this massive displacement of the sea floor as well that gave that bigger-than-expected tsunami."

Toy says this is the first time such a massive displacement of the sea floor during an earthquake has been seen, although scientists have suspected a lot of major faults aren't as strong as previously thought.

While the impact on future quakes at the same site is uncertain, she says investigation of other undersea subduction zones is important if we are to be adequately prepared for tsunamis in the future.

Scientists need to revisit their understanding of similar under-water subduction zones, for example off Chile, northern New Zealand and Alaska, she says.

"All of these are large subduction zones capable of generating tsunami and we probably need to revise our estimates of the size of tsunami that can be generated. They are likely to be larger than expected," she says.